A new genetic study of polar bears and brown bears led by researchers at the University of California, Santa Cruz has overturned prevailing ideas about the evolutionary history of the two species.

ADVERTISEMENT

Brown bears and polar bears are closely related and known to produce fertile ursid hybrids. Previous studies suggested that past hybridization had resulted in all polar bears having genes that came from brown bears. But new research indicates that episodes of gene flow between the two species occurred only in isolated populations and did not affect the larger polar bear population, which remains free of brown bear genes.

Research reveals that at the end of the last ice age, a population of polar bears was stranded by the receding ice on a few islands in southeastern Alaska. Male brown bears swam across to the islands from the Alaskan mainland and mated with female polar bears, eventually transforming the polar bear population into brown bears.

The genetic debate stems from a population of brown bears that live on Alaska's Admiralty, Baranof, and Chicagof Islands, known as the ABC Islands. These bears that are clearly brown bears in appearance and behavior have genetic similarities closer to polar bears.

"This population of brown bears stood out as being really weird genetically, and there's been a long controversy about their relationship to polar bears. We can now explain it, and instead of the convoluted history some have proposed, it's a very simple story," said coauthor Beth Shapiro, associate professor of ecology and evolutionary biology at UC Santa Cruz.

Shapiro and her colleagues analyzed genome-wide DNA sequence data from seven polar bears, an ABC Islands brown bear, a mainland Alaskan brown bear, and a black bear. Shapiro's team found that polar bears are a remarkably homogeneous species with no evidence of brown bear ancestry, whereas the ABC Islands brown bears show clear evidence of polar bear ancestry.

"The combination of genetics and the known behavior of brown and polar bears hybridizing in the wild today tells us how the ABC Islands bears came to be: they are the descendants of many male brown bear immigrants and some female polar bears from long ago," Stirling said.

As climate warms and arctic sea is lost, this scenario may happen on a broader scale. Only time will tell if polar bear genes will still be present in the brown bear genomes.